You will probably find the streets have been dug up by CityFibre and Vodafone have an agreement for "1st Dibs" on customers. Usually with all these deals at renewal time a threat to go back to BT or someone else (CityFibre may be offering their own services by this point or they may have another wholesale customer) usually gets renewal at the same price, in return for another 12 or 24 month contract.
Fibre to the Home (FTTH) is more than just a higher bandwidth. Your BT line is copper (or sometimes a bit of aluminium) that corrodes and joints get wet. The signal is electric. Hence the service can vary greatly and you get more faults. FTTH means an optical fibre direct to you home router and water has no effect. It doesn't really degrade much over time (25 years anyway). And the bandwidth can be increased almost without limit. It's worth remembering that bandwidth usage has been doubling every 18 months (since the 1980s, so while 100Mbps might seem good today, in 5 years you will want 800Mbps.